Make sure splash screen doesn't block supplying an engine to FlutterViewController. #24260
Description
FlutterViewController will attempt to show a splash screen using the default one for the app if it can't find one.
This is undesirable in many Add2App use cases where someone wants to display a FlutterViewController as a child/sub view without using a splash screen, but still has a splash screen for their app.
The embedding API should offer some easier way to avoid this.